Arthritis of the knee joint is not only painful but can be permanently debilitating. With ever increasing frequency, doctors are replacing arthritic knees with prosthetic devices having a tibial, a femur and a patella component which mimic the articulation between the tibia and the femur. A complete knee replacement is often referred to as a Total Knee Arthroplasty (TKA). It is a primary goal of TKA to provide a stable, pain free and long lasting knee replacement.
There are traditionally two types of TKA. The first type involves mechanically linking the tibial and femur components of the prosthesis which is termed a "constrained" prothesis. In the second type, the tibial and femur components are not mechanically linked but rather are physiologically stabilized with a patient's muscle and ligaments. This prothesis type is commonly known as a "unconstrained" prothesis, and comprises the overwhelming majority of current knee replacements. It is not surprising, based on the popularity of knee replacement surgery, that the United States patent literature is replete with examples of both constrained and unconstrained knee replacement prothesis. For example, U.S. Pat. Nos. 4,358,859 and 4,462,120 are representative of constrained prothesis. U.S. Pat. Nos. 4,309,778 and 4,470,158 are representative of unconstrained prothesis.
Fixation of the tibia, femur and patella components of the prothesis during implantation has customarily involved either bone cement or natural bone ingrowth. Orthopedic surgeons typically prefer cementless fixation for what is considered to be its potential to provide long term implant stability. Knee replacement prothesis wherein fixation is accomplished through bone ingrowth may have a porous layer to facilitate bone ingrowth. For instance, U.S. Pat. No. 4,479,271 discloses a tibial component which utilizes a fibrous metal mesh layer to facilitate bone ingrowth. U.S. Pat. Nos. 3,605,123; 3,855,638; and 4,550,448 further disclose porous layers which aid in the development of bone ingrowth.
Three factors are thought of as important for achieving optimal bone ingrowth: (1) close contact between bone and the prothesis, (2) the absence of micromotion and (3) the elimination of any effect which would inhibit bone growth. (see, Voltz, R. G.; Nisbet, J. K.; Lee, R. W.; and McMurtry, M. G.: The Mechanical Stability of Various Noncemented Tibial Components., Clin. Orthop. and Rel. Res., 226:38-42, 1988). Loosening due to micromotion of the tibial component is the most frequent cause of long term TKA failure. Porous coated implant designs were thought to have solved the loosening problem by providing a stable implant fixation through bony ingrowth. (see, Shimakagi, H.; Bechtold, J. E.; Sherman, R. E.; and Gustilo, R. B.: Stability of the Tibial Component in Cementless Total Knee Arthroplasty., J. of Orthopaedic Res., 8:64-71, 1990). However, cancellous bone ingrowth in the tibial prosthesis has been unpredictable. (see, Branson, P. J.; Steege, J. W.; Wixson, R. L.; Lewis, J.; and Stulberg, S. D.: Rigidity of Initial Fixation with Uncemented Tibial Knee Implants., J. of Arthroplasty, 4:21-26, 1989; Miura, H.; Whiteside, L. A.; Easley, J. C.; and Amador, D. D.: Effect of Screws and a Sleeve on Initial Fixation in Uncemented Total Knee Tibial Components., Clin. Orthop. and Rel. Res., 259:160-168, 1990; and Voltz, R. G.; Nisbet, J. K.; Lee, R. W.; and McMurtry, M. G.: The Mechanical Stability of Various Noncemented Tibial Components., Clin. Orthop. and Rel. Res., 226:38-42, 1988).
Testing of different tibial configurations, such as peripherally placed cancellous screws, sleeves, central stems and pegs, has shown that some resistance to micromotion can be obtained. (see, Branson, P. J.; Steege, J. W.; Wixson, R. L.; Lewis, J.; and Stulberg, S. D.: Rigidity of Initial Fixation with Uncemented Tibial Knee Implants., J. of Arthroplasty, 4:21-26, 1989; Cameron, H. U.: Noncemented Tibial Components: Does a Stem Help?, Contemporary Orthopaedics, 24:326-330, 1992; Lee, R. W.; Voltz, R. G.; and Sheridan. D. C.: The Role of Fixation and Bone Quality on the Mechanical Stability of Tibial Knee Components., Clin. Orthop. and Rel. Res., 259:160-168, 1990; Miura, H.; Whiteside, L. A.; Easley, J. C.; and Amador, D. D.: Effect of Screws and a Sleeve on Initial Fixation in Uncemented Total Knee Tibial Components., Clin. Orthop. and Rel. Res., 259:160-168, 1990; Shimakagi, H.; Bechtold, J. E.; Sherman, R. E.; and Gustilo, R. B.: Stability of the Tibial Component in Cementless Total Knee Arthroplasty., J. of Orthopaedic Res., 8:64-71, 1990; and Voltz, R. G.; Nisbet, J. K.; Lee, R. W.; and McMurtry, M. G.: The Mechanical Stability of Various Noncemented Tibial Components., Clin. Orthop. and Rel. Res., 226:38-42, 1988). The reason for inconsistent results with ingrowth stems from the inability for implant designs to provide adequate initial fixation in order for bone growth to occur. (see, Bindeglass, D. F.; Cohen, J. L.; and Dorr, L. D.: Current Principles of Design for Cemented and Cementless Knees., Techniques Orthop., 6:80-85, 1991; and Miura, H.; Whiteside, L. A.; Easley, J. C.; and Amador, D. D.: Effect of Screws and a Sleeve on Initial Fixation in Uncemented Total Knee Tibial Components., Clin. Orthop. and Rel. Res., 259:160-168, 1990).
Cemented TKA prosthesis designs have met with more clinical success and a lower incidence of loosening due to micromotion. (see, Krackow, K. A.; Hungeford, D. S.; Trnka, H. J.; Maar, D. C.; Mont, M. A.; and Urquhart, M.: Cemented Versus Uncemented Primary Total Knee Arthroplasty: A Comparative Study of the First 100 Patients in Each Group., Read at the Annual Meeting of the American Academy of Orthopaedic Surgeons, Washington D.C., Feb. 20, 1992; Insall, J. N.; Binazzi, R.; Soudry, M.; and Mestriner, L. A.: Total Knee Arthroplasty., Clin. Orthrop. and Rel. Res., 192:13-22, 1985; and Walker, P.S.: Requirements for Successful Total Knee Replacements, Orthrop., Clin. of North Am., 20:15-29, 1989). Yet, even bone cement fixated prosthetic components are susceptible to loosening.
Hybrid knee replacement prosthetic components have been developed in an attempt to overcome some of the above described disadvantages of the cementless and bone cement prosthesis. Hybrid components utilize both cement and bone ingrowth for fixation. An example of such a hybrid tibial component is disclosed in U.S. Pat. No. 4,938,769 to Shaw. The Shaw patent teaches a removable tibial tray component having a bone anchorage assembly including a central stem and two pegs. The central stem is designed to fit within the intramedullary canal, and the pegs, which are spaced symmetrically from the central stem, are structured to fit within the posterior-lateral and posterior-medial quadrants of the tibia. The central stem and pegs may contain an area coated with a porous metal to promote bone ingrowth. The Shaw patent further teaches that the distal end of the tibial tray may have a narrow, axially raised flange positioned on its periphery. The flange provides an area for the application of bone cement when the tibial tray is positioned on the tibia. In a preferred embodiment, the distal side of the tibial tray has one or more walled recesses which may be angled acutely and which serve to fixate the tibial tray to the tibia through the use of bone cement. The Shaw patent teaches that immediate fixation via the bone cement encourages permanent fixation of the prosthesis via bone ingrowth by minimizing motion between the bone surface and the prothesis.
The Shaw tibial prosthesis suffers from disadvantages primarily resulting from the placement of the porous bone ingrowth material on the central stem and pegs. The central stem and pegs of the Shaw prosthesis are structured to fit within the tibia itself and promote bone ingrowth between the bone tissue of the interior of the tibia and the central stem and pegs. While this design would appear to effectively promote bone ingrowth, because bone ingrowth occurs about the central stem and pegs, removal of the tibial prosthesis (as a result of infection) would cause excessive tibia damage. Moreover, the bone ingrowth area is of limited surface area and does not effectively utilize the larger surface area of the upper end of the tibia for bone ingrowth.
In addition, if bone ingrowth occurs on the stem as designed, physiologic loading of the proximal tibia may no longer occur. (Normally, the proximal tibia transmits all the forces placed on the leg.) Bone ingrowth occurring at a point from the proximal tibia surface can have the effect of bypassing the normal stress transfer of the tibia and lead to a situation of "stress-shielding" the proximal tibia surface. This phenomenon is well described in femoral prostheses used in hip replacements which obtain bone ingrowth in the femoral canal a distance away from the proximal femur (see Engh, Charles A.; Glassman, Andrew H.; and Suthers, Kathleen E.: The Case for Porous-Coated Hip Implants., Clin. Orthrop. and Rel. Res., 261:63-81, 1990; Harris, William H.: Will Stress Shielding Limit the Longevity of Cemented Femoral Components of Total Hip Replacement, Clin. Orthrop. and Rel. Res., 274:120-123, 1992; and Huiskes, Rik.; Weinans, Harrie.; and Van Rietbergen, Bert.: The Relationship Between Stress Shielding and Bone Resorption Around Total Hip Stems and the Effects of Flexible Materials., Clin. Orthrop. and Rel. Res., 274:124-134, 1992). Although proponents of this prosthetic design consider this acceptable (albeit a less than optimal situation), most surgeons avoid this design because of possible long term deleterious effects such as severe weakening of the proximal bone leading to fractures around the prosthesis.
Finally, a most concerning problem of the two piece design of Shaw arises from the locking screws. The Shaw design assumes that both permanent cement fixation and bone ingrowth will occur every time the device is implanted. If bone ingrowth occurs on the stem and the cement suffers loss of fixation (either by stress-shielding described previously or by the usual slower cement loosening process) then a situation exists whereby the two piece design will be greatly stressed. The proximal plate can toggle on the well-fixed stem leading to problems with metallic wear debris from the opposing two surfaces and from the locking screws. Metallic wear debris from screws used to secure cementless tibial trays and cementless acetabular hip protheses has been previously documented. (see Engh, Gerard A.; Smith, Nancy L.; and Ammeen, Deborah J.: Tibial Osteolysis in Cementless Total Knee Arthroplasty: A Review of 23 Tibial Lesions Surgically Treated With And Without Tibial Component Retention., Presented at The Knee Society Scientific Meeting, New Orleans, La., Feb. 27, 1994; and Whiteside, Leo A.; Four Screws for Fixation of the Tibial Component in Cementless Total Knee Arthroplasty., Clinical Orthrop. and Rel. Res., 299:72- 76, 1994).